I have ubuntu 11.10 , and I installed kde and I choosed kdm mistakly, as mentioned here I did this procedure

Pretty simple. Open the file /etc/X11/default-display-manager with your editor of choice. Make sure you invoke that editor as root, otherwise it won't work. In that file there's a single line: /usr/bin/kdm Change that to /usr/bin/gdm and save the file. Reboot and you're in Gnome.

now ,after booting, the log in manager gdm or kdm didn't appear .
how can I fix this?